1. Field of the Invention
The present invention relates to an image surface illuminance varying apparatus, an exposure correcting apparatus and an exposure correcting method, for a photographing apparatus, such as a single lens reflex camera or a compact camera, having an optical element whose transmittance varies depending on the position at which light is transmitted through the optical element provided in a photographing optical system thereof.
2. Description of the Related Art
Generally, in a lens of a camera, a light-quantity ratio (illuminance ratio/specific image surface illuminance) between a central portion and a peripheral portion of an image surface is increased toward the wide-angle side (short focal length side). Namely, the illuminance at the central portion of the image surface is high and the illuminance of the peripheral portion thereof is low. In an ultra wide-angle lens, since the light-quantity ratio is very high, it is known for a correction filter to be used to reduce the light-quantity ratio. The correction filter has a transmittance which is continuously varied (gradation) from the paraxial area (center portion) of the filter toward the peripheral portion thereof. The transmittance of the filter in the paraxial area is smaller than that at the peripheral area. If the correction filter is used in an ultra wide-angle lens, the light-quantity ratio between the central portion and the peripheral portion of the image surface is reduced (see Reference No. 1: Japanese Unexamined Publication No. 2004-258494).
In a single lens reflex camera or a compact camera having such a correction filter, the exposure is determined basically in accordance with a photometering measurement (brightness) of an object to thereby determine the diaphragm value and/or shutter speed. The combination of the diaphragm value and the shutter speed always abides by a specific law. For example, according to the APEX system, the exposure Ev, the diaphragm value Av and the shutter speed Tv abide by the following formula:Ev=Av+Tv 
However, if a correction filter is used with a zoom lens, the zoom lens of a camera exhibits, in general, a large light-quantity ratio (illuminance difference) between the central portion and the peripheral portion of the image surface at a wide-angle (short focal length) and the light-quantity ratio is decreased toward the telephoto side (long focal length). Therefore, in a known light-quantity ratio varying device, the correction in the quantity of light depends considerably on the focal length which is varied during the zooming operation.
In a photographing lens having an optical element whose transmittance varies depending on the position at which light is transmitted through the optical element, a change of the image surface illuminance is not linear with respect to a change in a control diaphragm value. Consequently, if combinations of the control diaphragm value Av and the shutter speed Tv obtained by calculation provide the same light quantity (exposure Ev), the actual light quantities could be different when the control diaphragm value Av corresponds to the open stop (minimum Av) compared to when the control diaphragm value corresponds to the minimum stop (maximum Av). Moreover, in case of a zoom lens, the image surface average illuminance can vary during the zooming operation depending on the position of the optical element, whose transmittance varies depending on the position at which light is transmitted therethrough. In this case, the actual light quantity may differ from the calculated light quantity (exposure Ev) depending on the control diaphragm value.